@qtiger, the Let's Encrypt service isn't publicly available yet. It will be publicly available in November. There isn't yet a way for people to obtain certificates from Let's Encrypt.
If you want to participate in the beta program, there's now some information available at
To participate in the beta program, you should be prepared for the possibility that some things will not work correctly at first, and be willing to help work with the project to investigate and fix the problems that might occur.